The present invention relates to methods and systems for selecting an antenna downtilt in a code division multiple access (CDMA) system, and to analysis of the reverse-link capacity of a cell-site.
In modern communication systems, there is a growing trend towards the multiple access technique known as code division multiple access (CDMA). In a code division multiple access system, a finite allocation of frequency, which may be the entire available frequency bandwidth or other amount of bandwidth as appropriate, is used by each call simultaneously. A transmission in a CDMA system includes a scrambled message and a code. Each call in progress has a unique code such that the receiver can pull the transmission out of the background noise by knowing the code for the call. For example, mobile clients may each be assigned a unique code on a per-call basis to use when transmitting.
CDMA is inherently an interference limited system, because the signal of every single mobile unit effectively contributes interference to the signals of all other mobile units. The total interference is due to mobiles being served by the same cell-site as well as mobiles being served by adjacent cell-sites. While the base station does need to receive signals from the mobiles being served by the same cell-site to maintain the respective links, undesired signals from mobiles being served by adjacent cell-sites generate interference. Generally, there are three different types of interference that are seen by the base station in the reverse-link from the mobile to the base station. The three interference types are same-sector interference, inter-sector interference, and out-of-cell interference. These three types of interference are seen by the antenna in a sector of a cell. That is, the network includes a plurality of cells, with each cell being made up of one or more sectors, three sectors being typical.
Same-sector interference is the interference seen by the sector antenna due to mobile devices within that sector. That is, signals from mobiles within the same sector contribute noise to each other. Inter-sector interference is the interference seen by the antenna due to mobile devices in other sectors of the same cell-site. That is, mobile devices within adjacent sectors may still fall within the antenna pattern of the sector of interest, thereby contributing noise. Lastly, out-of-cell interference is the interference seen by the antenna that comes from mobile devices in other cells, such as adjacent cells.
There is a need for a method and system that provides appropriate interference rejection without unduly compromising CDMA network performance.
It is, therefore, an object of the present invention to provide a method for controlling downtilt of an antenna in a sector of a cell of a code division multiple access (CDMA) network for mobile devices. The method comprises monitoring current network conditions and estimating relative sector capacity at a plurality of antenna downtilts for the current network conditions. The method further comprises selecting an antenna downtilt based on the relative sector capacity at the plurality of antenna downtilts and dynamically applying the selected downtilt to the antenna.
Preferably, the method further comprises estimating relative external interference rejection and relative sectorization efficiency. Preferably, the antenna downtilt is selected based on the relative external interference rejection, the relative sectorization efficiency, and the relative sector capacity at the plurality of antenna downtilts. Further, preferably, the method comprises estimating a percentage of mobile devices within the sector that are in a softer handoff region defined by a softer handoff margin between sectors within the same cell.
In a preferred embodiment, the estimated relative external interference rejection is based on a loading factor that is a ratio of the out-of-cell interference to same-sector interference. Further, in a preferred embodiment, the estimated relative sectorization efficiency is based on a ratio of a sum of same-sector interference and out-of-cell interference to a sum of the same-sector interference, the inter-sector interference, and the out-of-cell interference.
Further, in carrying out the present invention, a method for controlling downtilt in a sector of a cell of a code division multiple access (CDMA) network for mobile devices is provided. The method comprises monitoring current network conditions, estimating relative external interference rejection, estimating relative sectorization efficiency, estimating relative sector capacity, selecting an antenna downtilt, and dynamically applying the selected downtilt to the antenna. The relative external interference rejection is estimated, based in part on out-of-cell interference, at a plurality of antenna downtilts for the current network conditions. The relative sectorization efficiency is estimated, based in part on inter-sector interference, at the plurality of antenna downtilts for the current network conditions. The relative sector capacity is based on the corresponding relative external interference rejection and sectorization efficiency at the plurality of antenna downtilts. The antenna downtilt is selected based on the relative sector capacity at the plurality of antenna downtilts, and is preferably selected to provide optimal or maximum relative sector capacity.
Still further, in carrying out the present invention, a method for controlling downtilt of an antenna in a sector of a cell of a code division multiple access (CDMA) network for mobile devices is provided. The method comprises monitoring current network conditions, estimating relative external interference rejection, estimating relative sectorization efficiency, estimating relative sector capacity, selecting the antenna downtilt, and dynamically applying the selected downtilt to the antenna.
The relative external interference rejection is estimated at a plurality of antenna downtilts for the current network conditions according to:
xe2x80x83IR(xcexa8)=1/(1+xcex7(xcexa8))
wherein xcexa8 is the downtilt angle, IR(xcexa8) is the estimated relative external interference rejection at downtilt angle xcexa8, and xcex7(xcexa8) is a loading factor at downtilt angle xcexa8.
The loading factor is determined according to:
xcex7(xcexa8)=Iext(xcexa8)/Iin(xcexa8)
wherein Iext (xcexa8) is out-of-cell interference at downtilt angle xcexa8 and Iin (xcexa8) is same-sector interference at downtilt angle xcexa8.
The relative sectorization efficiency is estimated according to:
Esec(xcexa8)=(Iin(xcexa8)+Iext(xcexa8))/(Iin(xcexa8)+Isec(xcexa8)+Iext(xcexa8))
wherein Eext (xcexa8) is the estimated relative sectorization efficiency at downtilt angle xcexa8 and Isec (xcexa8) is the inter-sector interference at downtilt angle xcexa8.
The relative sector capacity at the plurality of antenna downtilt for the current network conditions is estimated based on a product of the corresponding relative external interference rejection and the corresponding relative sectorization efficiency. That is, relative sector capacity is preferably estimated according to:
Csec(xcexa8)=(1/(1+xcex7(xcexa8)))Esec(xcexa8)
wherein Csec (xcexa8) is the estimated relative sector capacity at downtilt angle xcexa8.
Yet further, in carrying out the present invention, a system for controlling downtilt of an antenna in a sector of a cell of a code division multiple access (CDMA) network for mobile devices is provided. The system comprises control logic configured to monitor current network conditions, estimate relative sector capacity, select an antenna downtilt, and dynamically apply the selected antenna downtilt to the antenna.
It is to be appreciated that the control logic may take any known form such as software, firmware, hardware, or combinations thereof. Further, the control logic may be located anywhere in the network, as appropriate for a particular application. For example, the control logic may be located at the base station, base station controller, mobile switching center, or may be located elsewhere in the network, for example, so as to be accessible by a network operations center which controls a plurality of base stations.
The advantages associated with embodiments of the present invention are numerous. For example, methods and systems of the present invention for controlling downtilt of an antenna in a sector of a cell of a code division multiple access (CDMA) network for mobile devices allow the antenna downtilt to be adjusted automatically as network conditions change. Antenna downtilt can be changed by either electrical or electro-mechanical downtilt. Further, it is to be appreciated that antenna downtilt may be changed based upon current network conditions on an effectively continuous basis.
The term xe2x80x9ceffectively continuousxe2x80x9d means that the antenna downtilt is adjusted to suit current network conditions on a repetitive basis, wherein the time between potential adjustments of antenna downtilt is fixed or variable. For example, antenna downtilt may be dynamically adjusted on a daily basis, on a weekly basis, or on an hourly basis. Further, antenna downtilt may be adjusted more often during times of extreme variations in network conditions than at times when network conditions are generally stable. Preferably, downtilt is adjusted or selected so as to effectively maximize cell-site capacity.
The above object and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.